The present invention relates to a corrosive environment sensor for measuring growth rate of a crack such as corrosion cracking occurring in a corrosive environment, a corrosive environment measuring apparatus for judging corrosive environmental conditions using the corrosive environment sensor and corrosive environment control system using the corrosive environment measuring apparatus, more particularly, it relates to a corrosive environment sensor of double cantilever type and a corrosive environment measuring apparatus, which are suitable to measure growth conditions of a crack in the corrosive environment sensor by a direct current potential method, calculate automatically a crack growth rate from change in the crack length to time and judge corrosion degree in the environment in which the corrosive environment sensor is placed.
In order to measure a crack growth rate in a corrosive environment, it is tentatively tried in a nuclear power plant to measure, by a direct current potential method, the length of a crack caused in a double cantilever type sensor having a load caused therein by applying a fixed displacement thereto. Such a double cantilever type sensor (hereinafter referred to as DCB sensor) has a pair of elongate beam portions opposed to each other and a crack growth portion formed between the beam portions to extend from an axially intermediate portion of each beam portion to a rear end thereof. The crack growth portion extends straightly with a proper width, and the width of the crack growth portion expands around the rear end to be the same as the width of the beam portion. A pre-cracking of a small length is formed at a crack starting end of the crack growth portion by electric discharge machining. The beam portions each have a wedge inserting portion formed around a front end opposite to the rear end to insert a wedge.
In the nuclear power plant, the DCB sensor is disposed in a place of a pressure vessel of a BWR where recirculation water flows. The wedge is inserted in the wedge inserting portion of the DCB sensor thereby to cause displacement in the beam portions and a load in the crack growth portion. Change of a crack occurred in the crack growth portion of the DBC sensor is measured and judged.
This kind of a conventional DCB sensor is disclosed in JP A 62-177440, JP A 2-259555, etc.
Although it is set forth that distribution of stress intensity factor in the conventional DCB sensor is uniform and although the sensor is so long as to be 7 inches in overall length, the DCB sensor has such characteristics that the crack growth of 1 inch reduces the stress intensity factor to 60% of the initial stress intensity factor, thereby giving rise to a disadvantage that the conventional DCB sensor is not suitable for measuring the crack growth rate with a proper stress intensity factor.
The inventors proposed previously a DCB sensor which is nearly uniform in the stress intensity factor distribution. A patent application concerning it was filed as U.S. Ser. No. 671,038.